Light-Emitting Element, Light-Emitting Device, and Display Device

ABSTRACT

A light-emitting element with which a reduction in power consumption and an improvement in productivity of a display device can be achieved is provided. A technique of manufacturing a display device with high productivity is provided. The light-emitting element includes an electrode having a reflective property, and a first light-emitting layer, a charge generation layer, a second light-emitting layer, and an electrode having a light-transmitting property stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement exhibiting electroluminescence, a display device including thelight-emitting element, and manufacturing methods of the light-emittingelement and the display device.

2. Description of the Related Art

In recent years, an electroluminescent (also referred to as EL) displaydevice has attracted attention as a display device with reducedthickness and weight (i.e., so-called flat panel display).

Light-emitting elements using light-emitting materials emitting light ofdifferent colors are provided as light-emitting elements used in pixelsin an EL display device, so that full-color display can be performed.

For such an EL display device, a method is used in which selectivedeposition of light-emitting materials in a minute pattern is performedfor each pixel by an evaporation method using a metal mask.

However, a shape defect or emission defect might be caused to alight-emitting element due to contact of a metal mask, and ways toprevent the defects have been explored (e.g., Patent Document 1). PatentDocument 1 discloses a structure in which a spacer for supporting ametal mask is provided over a pixel electrode so that the metal mask andthe pixel electrode are not in contact with each other at the time ofevaporation.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-126817

SUMMARY OF THE INVENTION

A method in which selective deposition of light-emitting materials isperformed for each pixel has a complicated process; thus, yield orproductivity is difficult to increase.

An object of one embodiment of the present invention is to provide alight-emitting element with which a reduction in power consumption andan increase in productivity of a display device can be achieved.

An object of one embodiment of the present invention is to provide atechnique for manufacturing a display device with high productivity.

An object of one embodiment of the present invention is to provide ahigh-definition display device with high color purity.

One embodiment of the present invention is a light-emitting elementincluding an electrode having a reflective property, and a firstlight-emitting layer, a charge generation layer, a second light-emittinglayer, and an electrode having a light-transmitting property which arestacked in this order over the electrode having a reflective property.The optical path length between the electrode having a reflectiveproperty and the first light-emitting layer is one-quarter of the peakwavelength of the emission spectrum of the first light-emitting layer.The optical path length between the electrode having a reflectiveproperty and the second light-emitting layer is three-quarters of thepeak wavelength of the emission spectrum of the second light-emittinglayer. Note that in this specification, a peak wavelength of an emissionspectrum refers to a wavelength of the highest emission intensity peakof a plurality of peaks in an emission spectrum.

In the above structure, the first light-emitting layer and the secondlight-emitting layer emit light of different colors.

One embodiment of the present invention is a light-emitting elementincluding an electrode having a reflective property, and a firstlight-emitting layer, a first charge generation layer, a secondlight-emitting layer, a second charge generation layer, a thirdlight-emitting layer, and an electrode having a light-transmittingproperty which are stacked in this order over the electrode having areflective property. The optical path length between the electrodehaving a reflective property and the first light-emitting layer isone-quarter of the peak wavelength of the emission spectrum of the firstlight-emitting layer. The optical path length between the electrodehaving a reflective property and the second light-emitting layer isthree-quarters of the peak wavelength of the emission spectrum of thesecond light-emitting layer. The optical path length between theelectrode having a reflective property and the third light-emittinglayer is five-quarters of the peak wavelength of the emission spectrumof the third light-emitting layer.

In the above structure, the first light-emitting layer, the secondlight-emitting layer, and the third light-emitting layer emit light ofdifferent colors.

One embodiment of the present invention is a display device including aplurality of light-emitting elements and a plurality of color filterlayers provided over the plurality of light-emitting elements. Each ofthe plurality of light-emitting elements includes an electrode having areflective property, and a first light-emitting layer, a chargegeneration layer, a second light-emitting layer, and an electrode havinga light-transmitting property which are stacked in this order over theelectrode having a reflective property. The optical path length betweenthe electrode having a reflective property and the first light-emittinglayer is one-quarter of the peak wavelength of the emission spectrum ofthe first light-emitting layer. The optical path length between theelectrode having a reflective property and the second light-emittinglayer is three-quarters of the peak wavelength of the emission spectrumof the second light-emitting layer. The plurality of color filter layersinclude at least a color filter layer which transmits light with a peakwavelength of the emission spectrum of the first light-emitting layerand a color filter layer which transmits light with a peak wavelength ofthe emission spectrum of the second light-emitting layer.

One embodiment of the present invention is a display device including aplurality of light-emitting elements and a plurality of color filterlayers provided over the plurality of light-emitting elements. Each ofthe plurality of light-emitting elements includes an electrode having areflective property, and a first light-emitting layer, a first chargegeneration layer, a second light-emitting layer, a second chargegeneration layer, a third light-emitting layer, and an electrode havinga light-transmitting property which are stacked in this order over theelectrode having a reflective property. The optical path length betweenthe electrode having a reflective property and the first light-emittinglayer is one-quarter of the peak wavelength of the emission spectrum ofthe first light-emitting layer. The optical path length between theelectrode having a reflective property and the second light-emittinglayer is three-quarters of the peak wavelength of the emission spectrumof the second light-emitting layer. The optical path length between theelectrode having a reflective property and the third light-emittinglayer is five-quarters of the peak wavelength of the emission spectrumof the third light-emitting layer. The plurality of color filter layersinclude at least a color filter layer which transmits light with a peakwavelength of the emission spectrum of the first light-emitting layer, acolor filter layer which transmits light with a peak wavelength of theemission spectrum of the second light-emitting layer, and a color filterlayer which transmits light with a peak wavelength of the emissionspectrum of the third light-emitting layer.

According to one embodiment of the present invention, a display devicecan be manufactured with high productivity.

Further, according to one embodiment of the present invention, ahigh-definition display device with high color purity can be provided.

Further, according to one embodiment of the present invention, a displaydevice with low power consumption can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views each illustrating a display device.

FIG. 2 is a view illustrating a display device.

FIGS. 3A to 3C are views illustrating display devices.

FIGS. 4A and 4B are views illustrating a display device.

FIGS. 5A to 5F are views each illustrating an example of application ofa display device.

FIG. 6 is a view illustrating a light-emitting element of Example.

FIG. 7 is a graph showing an emission spectrum of the light-emittingelement of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to theaccompanying drawings. Note that the present invention is not limited tothe following description, and it will be easily understood by thoseskilled in the art that various changes and modifications can be madewithout departing from the spirit and scope of the present invention.Therefore, the present invention should not be construed as beinglimited to the description of the embodiments below. In the structuresto be given below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and explanation thereof will not be repeated.

Embodiment 1

In this embodiment, one embodiment of an EL display device will bedescribed with reference to FIGS. 1A and 1B, FIG. 2, and FIGS. 3A to 3C.

FIG. 1A illustrates a structure of a light-emitting element according tothis embodiment. The light-emitting element illustrated in FIG. 1Aincludes an electrode 102 having a reflective property, and a first ELlayer 106, a charge generation layer 107, a second EL layer 110, and anelectrode 112 having a light-transmitting property which are stacked inthis order over the electrode 102 having a reflective property. Lightemitted from the light-emitting element is emitted from the electrode112 side.

In FIG. 1A, the first EL layer 106 includes at least a firstlight-emitting layer 120. The second EL layer 110 includes at least asecond light-emitting layer 122. Note that each of the first EL layer106 and the second EL layer 110 can have a stacked-layer structureincluding functional layers such as a hole-injection layer, ahole-transport layer, an electron-transport layer, and anelectron-injection layer, in addition to the light-emitting layer.

Light emitted from the first light-emitting layer 120 in each of thelight-emitting elements illustrated in FIGS. 1A and 1B is extractedoutside the light-emitting element through the electrode 112 having alight-transmitting property. Here, components of light entering theelectrode 112 having a light-transmitting property include lightentering the electrode 112 having a light-transmitting property directlyfrom the first light-emitting layer 120 (the light is also referred toas first entering light) and light which is emitted from the firstlight-emitting layer 120 and is reflected back by the electrode 102having a reflective property (the light is also referred to as firstreflected light). The first entering light and the first reflected lightinterfere with each other. Thus, the optical path length between theelectrode 102 having a reflective property and the first light-emittinglayer 120 is adjusted, whereby the phase of the first entering light andthe phase of the first reflected light can be aligned with each other.

In this embodiment, the optical path length between the electrode 102having a reflective property and the first light-emitting layer 120 isset to one-quarter of the peak wavelength of the emission spectrum ofthe first light-emitting layer. This allows the phases of the firstentering light and the first reflected light to be aligned and emissionfrom the first light-emitting layer 120 to be increased. Thus, theluminance of the light-emitting element according to this embodiment ishigher than the luminance of a light-emitting element in which theoptical path length is not adjusted, in the case where the same currentis applied to these light-emitting elements.

In a similar manner, light emitted from the second light-emitting layer122 is extracted outside the light-emitting element through theelectrode 112 having a light-transmitting property. Components of lightentering the electrode 112 having a light-transmitting property includelight entering the electrode 112 having a light-transmitting propertydirectly from the second light-emitting layer 122 (the light is alsoreferred to as second entering light) and light which is emitted fromthe second light-emitting layer 122 and is reflected back by theelectrode 102 having a reflective property (the light is also referredto as second reflected light). The second entering light and the secondreflected light interfere with each other. Thus, the optical path lengthbetween the electrode 102 having a reflective property and the secondlight-emitting layer 122 is adjusted, whereby the phase of the secondentering light and the phase of the second reflected light can bealigned with each other.

In this embodiment, the optical path length between the electrode 102having a reflective property and the second light-emitting layer 122 isset to three-quarters of the peak wavelength of the emission spectrum ofthe second light-emitting layer. This allows the phases of the secondentering light and the second reflected light to be aligned and emissionfrom the second light-emitting layer 122 to be increased. Thus, theluminance of the light-emitting element according to this embodiment ishigher than the luminance of a light-emitting element in which theoptical path length is not adjusted, in the case where the same currentis applied to these light-emitting elements.

More strictly, the optical path length between the electrode 102 havinga reflective property and the first light-emitting layer 120 can also bereferred to as the optical path length between the electrode 102 havinga reflective property and a light-emitting region in the firstlight-emitting layer 120. Note that it is difficult to strictlydetermine the position of the light-emitting region in thelight-emitting layer and that the effects described above can besufficiently obtained by assuming any position in the light-emittinglayer as the position of the light-emitting region. In other words, theoptical path length between the electrode 102 having a reflectiveproperty and the first light-emitting layer 120 can be referred to asthe optical path length between a surface of the electrode 102 having areflective property and a lower surface of the first light-emittinglayer 120 or more and the optical path length between the surface of theelectrode 102 having a reflective property and an upper surface of thefirst light-emitting layer 120 or less. The same can be applied to theoptical path length between the electrode having a reflective property102 and the second light-emitting layer 122, and the optical path lengthbetween the electrode 102 having a reflective property and the thirdlight-emitting layer 124 which will be described later.

Note that it is preferable that the first light-emitting layer 120 andthe second light-emitting layer 122 emit light of different colors.

The structure of the light-emitting element illustrated in FIG. 1A willbe described below along with specific materials. Note that an elementstructure, a manufacturing method, and the like described here are justexamples, and other known structures, materials, and manufacturingmethods can be applied without departing from the purpose of thisembodiment.

The electrode 102 having a reflective property is provided opposite tothe side where light is extracted and is formed using a reflectivematerial. As the reflective material, a metal material such as aluminum,gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron,cobalt, copper, or palladium can be used. In addition, any of thefollowing can be used: alloys containing aluminum (aluminum alloys) suchas an alloy of aluminum and titanium, an alloy of aluminum and nickel,and an alloy of aluminum and neodymium; and an alloy containing silversuch as an alloy of silver and copper. An alloy of silver and copper ispreferable because of its high heat resistance. Further, a metal film ora metal oxide film is stacked on an aluminum alloy film, wherebyoxidation of the aluminum alloy film can be prevented. As examples of amaterial of the metal film or the metal oxide film, titanium, titaniumoxide, and the like are given. The above materials are preferablebecause they are present in large amounts in the Earth's crust andinexpensive to achieve a reduction in manufacturing cost of alight-emitting element.

In this embodiment, the case where the electrode 102 having a reflectiveproperty is used as an anode of the light-emitting element is describedas an example. However, one embodiment of the present invention is notlimited thereto.

A conductive layer having a light-transmitting property may be providedon and in contact with the electrode 102 having a reflective property sothat the optical path length between the electrode 102 having areflective property and the first light-emitting layer 120. Theconductive layer having a light-transmitting property is formed of asingle layer or stacked layers using a material having a property oftransmitting visible light. As the light-transmitting material, forexample, indium oxide, indium tin oxide, indium oxide-zinc oxide, zincoxide, zinc oxide to which gallium is added, graphene, or the like canbe used.

The conductive layer having a light-transmitting property can be formedusing a conductive composition containing a conductive high molecule(also referred to as conductive polymer). As the conductive highmolecule, a so-called π-electron conjugated conductive polymer can beused. For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene (PEDOT) or a derivative thereof, acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof, and the like can be given.

The first EL layer 106 may include at least the first light-emittinglayer 120. In addition, the first EL layer 106 can have a stacked-layerstructure in which a layer containing a substance having a highhole-transport property, a layer containing a substance having a highelectron-transport property, a layer containing a substance having ahigh hole-injection property, a layer containing a substance having ahigh electron-injection property, a layer containing a bipolar substance(a substance having a high hole-transport and electron-transportproperties), and the like are combined as appropriate. For example, thefirst EL layer 106 can have a stacked-layer structure including ahole-injection layer, a hole-transport layer, the first light-emittinglayer 120, an electron-transport layer, and an electron-injection layer.Needless to say, in the case where the electrode 102 having a reflectiveproperty is used as a cathode, a stacked-layer structure in which anelectron-injection layer, an electron-transport layer, the firstlight-emitting layer 120, a hole-transport layer, and a hole-injectionlayer are stacked in this order from the cathode side may be employed.

The hole-injection layer is a layer containing a substance having a highhole-injection property. As the substance having a high hole-injectionproperty, for example, metal oxides such as molybdenum oxide, titaniumoxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide,zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungstenoxide, and manganese oxide can be used. A phthalocyanine-based compoundsuch as phthalocyanine (abbreviation: H₂Pc), or copper(II)phthalocyanine (abbreviation: CuPc) can also be used.

Any of the following aromatic amine compounds which are low molecularorganic compounds can also be used:4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Any of the high molecular compounds (e.g., oligomers, dendrimers, orpolymers) can also be used. Examples of the high molecular compoundinclude poly(N-vinylcarbazole) (abbreviation: PVK),poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD). A high molecular compound to which acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)or polyaniline/poly(styrenesulfonic acid) (PAni/PSS), can also be used.

In particular, it is preferable to use a composite material in which anacceptor substance is mixed with an organic compound having a highhole-transport property for the hole-injection layer. With the use ofthe composite material in which an acceptor substance is mixed with asubstance having a high hole-transport property, excellent holeinjection from the anode can be obtained, which results in a reductionin the driving voltage of the light-emitting element. Such a compositematerial can be formed by co-evaporation of a substance having a highhole-transport property and a substance having an acceptor property.When the hole-injection layer is formed using the composite material,holes are easily injected into the first EL layer 106 from the anode.

As the organic compound for the composite material, a variety ofcompounds such as an aromatic amine compound, carbazole derivatives,aromatic hydrocarbon, and a high molecular compound (such as oligomer,dendrimer, or polymer) can be used. The organic compound used for thecomposite material is preferably an organic compound having a highhole-transport property. Specifically, a substance having a holemobility of 10⁻⁶ cm²/Vs or higher is preferably used. Note that anyother substances may also be used as long as the hole-transport propertythereof is higher than the electron-transport property thereof. Specificexamples of the organic compound that can be used for the compositematerial will be given below.

As the organic compound that can be used for the composite material, anyof the following can be used: aromatic amine compounds such as TDATA,MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),and carbazole derivatives, such as 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Any of the following aromatic hydrocarbon compounds can also be used:2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene, and2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Any of the following aromatic hydrocarbon compounds can also be used:2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), or 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

Examples of the electron acceptor include organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil; and transition metal oxides. Other examplesinclude oxides of metals belonging to Groups 4 to 8 in the periodictable. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is particularly preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

Note that the hole injection layer may be formed using a compositematerial of the high molecular compound such as PVK, PVTPA, PTPDMA, orPoly-TPD, and the electron acceptor.

Note that in the case where a layer containing the above compositematerial is provided in the first EL layer 106, the optical path lengthbetween the electrode 102 having a reflective property and the firstlight-emitting layer 120 (the optical path length of the first reflectedlight) may be adjusted by adjusting the thickness of the layercontaining the above composite material. In that case, the conductivelayer having a light-transmitting property is not necessarily provided.

The hole-transport layer is a layer containing a substance having a highhole-transport property. As the substance having a high hole-transportproperty, any of the following aromatic amine compounds can be used, forexample: NPB, TPD, BPAFLP,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). The substances given here are mainly ones having ahole mobility of 10⁻⁶ cm²/Vs or higher. However, any other substancesmay also be used as long as the hole-transport property thereof ishigher than the electron-transport property thereof. The layercontaining a substance having a high hole-transport property is notlimited to a single layer, and may be a stack of two or more layerscontaining any of the above substances.

A carbazole derivative such as CBP, CzPA, or PCzPA or an anthracenederivative such as t-BuDNA, DNA, or DPAnth may be used for thehole-transport layer.

Alternatively, a high molecular compound such as PVK, PVTPA, PTPDMA, orPoly-TPD can be used for the hole-transport layer.

The first light-emitting layer 120 is a layer containing alight-emitting organic compound. As the light-emitting organic compound,for example, a fluorescent compound which emits fluorescence or aphosphorescent compound which emits phosphorescence can be used.

As the fluorescent compound that can be used for the firstlight-emitting layer 120, a material for blue light emission, a materialfor green light emission, a material for yellow light emission, and amaterial for red light emission are given. Examples of the material forblue light emission includeN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA). Example of the material for green light emissioninclude N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), and N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA). Examples of the material for yellow lightemission include rubrene and5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT).Examples of the material for red light emission includeN,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD) and7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD).

As the phosphorescent compound that can be used for the firstlight-emitting layer 120, a material for blue light emission, a materialfor green light emission, a material for yellow light emission, amaterial for orange light emission, a material for red light emissionare given. Examples of the material for blue light emission includebis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)), andbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(abbreviation: FIr(acac)). Examples of the material for green lightemission include tris(2-phenylpyridinato-N,C^(2′))iridium(III)(abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)), andtris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃).

Examples of the material for yellow light emission includebis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-(perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(III)(abbreviation: Ir(Fdppr-Me)₂(acac)), and(acetylacetonato)bis{2-(4-methoxyphenyl)-3,5-dimethylpyrazinato}iridium(III)(abbreviation: Ir(dmmoppr)₂(acac)). Examples of the material for orangelight emission include tris(2-phenylquinolinato-N,C^(2′))iridium(III)(abbreviation: Ir(pq)₃),bis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(pq)₂(acac)),(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)), and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)). Examples of the material for redlight emission include organometallic complexes such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′))iridium(III)acetylacetonate(abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),(dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)), and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II)(abbreviation: PtOEP). In addition, rare earth metal complexes, such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)), tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)),exhibit light emission from rare earth metal ions (electron transitionbetween different multiplicities), and thus can be used asphosphorescent compounds.

Note that the first light-emitting layer 120 may have a structure inwhich any of the above light-emitting organic compounds (a guestmaterial) is dispersed in another substance (a host material). As a hostmaterial, various kinds of materials can be used, and it is preferableto use a substance which has a lowest unoccupied molecular orbital level(LUMO level) higher than the light-emitting substance and has a highestoccupied molecular orbital level (HOMO level) lower than that of thelight-emitting substance.

As the host material, specifically, any of the following can be used:metal complexes such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), andbathocuproine (BCP); condensed aromatic compounds such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3),9,10-diphenylanthracene (abbreviation: DPAnth), and6,12-dimethoxy-5,11-diphenylchrysene; and aromatic amine compounds suchas N,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1A), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB, TPD, DFLDPBi, and BSPB.

Plural kinds of materials can be used as the host material. For example,in order to suppress crystallization, a substance such as rubrene whichsuppresses crystallization, may be further added. In addition, NPB, Alq,or the like may be further added in order to efficiently transfer energyto the guest material.

When the structure in which a guest material is dispersed in a hostmaterial is employed, crystallization of the first light-emitting layer120 can be suppressed. In addition, concentration quenching due to highconcentration of the guest material can be suppressed.

A high molecular compound can be used for the first light-emitting layer120. As specific examples of the high molecular compound, a material forblue light emission, a material for green light emission, and a materialfor orange to red light emission are given. Examples of the material forblue light emission include poly(9,9-dioctylfluorene-2,7-diyl)(abbreviation: PFO),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: PF-DMOP), andpoly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH). Examples of the material for green lightemission include polyp-phenylenevinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)](abbreviation: PFBT), andpoly[(9,9-dioctyl-2,7-divinylenfluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)].Examples of the material for orange to red light emission includepoly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation:MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation: R4-PAT),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}, andpoly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD).

Note that the first EL layer 106 may have a structure including two ormore light-emitting layers.

The electron-transport layer is a layer containing a substance having ahigh electron-transport property. As the substance having a highelectron-transport property, any of the following can be used, forexample: metal complexes having a quinoline skeleton or a benzoquinolineskeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), andbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq). A metal complex or the like including an oxazole-based orthiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc(abbreviation: Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(abbreviation: Zn(BTZ)₂) can also be used. Other than the metalcomplexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances given here are mainly ones having an electron mobility of10⁻⁶ cm²/Vs or higher. The electron-transport layer is not limited to asingle layer and may be a stack of two or more layers containing any ofthe above substances.

The electron-injection layer is a layer containing a substance having ahigh electron-injection property. For the electron-injection layer, analkali metal, an alkaline-earth metal, or a compound thereof, such aslithium, cesium, calcium, lithium fluoride, cesium fluoride, calciumfluoride, or lithium oxide, can be used. Alternatively, a rare earthmetal compound such as erbium fluoride can be used. Furtheralternatively, any of the above substances for forming theelectron-transport layer can be used.

Charges are generated in the charge generation layer 107 by applyingvoltage to the light-emitting element. The charge generation layer 107has functions of injecting holes into the EL layer on the cathode sideand injecting electrons into the EL layer on the anode side.

The charge generation layer 107 can be formed using the above compositematerial. The charge generation layer 107 may have a stacked-layerstructure including a layer containing the composite material and alayer containing another material. In that case, as the layer containinganother material, a layer containing an electron donating substance anda substance with high electron-transport properties, a layer formed of atransparent conductive film, or the like can be used. As for alight-emitting element having such a structure, problems such as energytransfer and quenching occur with difficulty, and a light-emittingelement which has both high light emission efficiency and long lifetimecan be easily obtained due to expansion in the choice of materials.Moreover, a light-emitting element which provides phosphorescence fromone EL layer and fluorescence from another EL layer can be easilyobtained.

Note that the optical path length between the electrode 102 having areflective property and the second light-emitting layer 122 (the opticalpath length of the second reflected light) can be adjusted by adjustingthe thickness of the charge generation layer 107.

When the charge generation layer is provided between the stacked ELlayers, the element can have high luminance and long lifetime while thecurrent density is kept low. In addition, a voltage drop due to theresistance of the electrode material can be reduced, whereby uniformlight emission in a large area is possible.

The second EL layer 110 may include at least the second light-emittinglayer 122. In addition, the second EL layer 110 can have a stacked-layerstructure in which a layer containing a substance having a highhole-transport property, a layer containing a substance having a highelectron-transport property, a layer containing a substance having ahigh hole-injection property, a layer containing a substance having ahigh electron-injection property, a layer containing a bipolar substance(a substance having a high hole-transport and electron-transportproperties), and the like are combined as appropriate. The second ELlayer 110 may have a structure similar to that of the first EL layer 106or may have a stacked-layer structure different from that of the firstEL layer 106. For example, the second EL layer 110 can have astacked-layer structure including a hole-injection layer, ahole-transport layer, the second light-emitting layer 122, anelectron-transport layer, an electron-injection buffer layer, anelectron-relay layer, and a composite material layer in contact with theelectrode 112 having a light-transmitting property. Note that the secondEL layer 110 may have a structure including two or more light-emittinglayers.

The composite material layer in contact with the electrode 112 having alight-transmitting property is preferably provided, in which case damagecaused to the second EL layer 110 particularly when the electrode 112having a light-transmitting property is formed by a sputtering methodcan be reduced. The composite material layer can be formed using theabove-described composite material in which an acceptor substance ismixed with an organic compound having a high hole-transport property.

Further, by providing the electron-injection buffer layer, an injectionbarrier between the composite material layer and the electron-transportlayer can be reduced; thus, electrons generated in the compositematerial layer can be easily injected into the electron-transport layer.

A substance having a high electron-injection property can be used forthe electron-injection buffer layer: for example, an alkali metal, analkaline earth metal, a rare earth metal, a compound of the above metal(e.g., an alkali metal compound (including an oxide such as lithiumoxide, a halide, or carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, or carbonate), or a rare earth metal compound (including anoxide, a halide, or carbonate).

In the case where the electron-injection buffer layer contains asubstance having a high electron-transport property and a donorsubstance, the donor substance is preferably added so that the massratio of the donor substance to the substance having a highelectron-transport property ranges from 0.001:1 to 0.1:1. Note that asthe donor substance, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be usedas well as an alkali metal, an alkaline earth metal, a rare earth metal,a compound of the above metal (e.g., an alkali metal compound (e.g., anoxide such as lithium oxide, a halide, and a carbonate such as lithiumcarbonate or cesium carbonate), an alkaline earth metal compound (e.g.,an oxide, a halide, and a carbonate), and a rare earth metal compound(e.g., an oxide, a halide, and a carbonate). Note that as the substancehaving a high electron-transport property, a material similar to thematerial for the electron transport layer described above can be used.

Furthermore, it is preferable that the electron-relay layer be formedbetween the electron-injection buffer layer and the composite materiallayer. The electron-relay layer is not necessarily provided; however, byproviding the electron-relay layer having a high electron-transportproperty, electrons can be rapidly transported to the electron-injectionbuffer layer.

The structure in which the electron-relay layer is sandwiched betweenthe composite material layer and the electron-injection buffer layer isa structure in which the acceptor substance contained in the compositematerial layer and the donor substance contained in theelectron-injection buffer layer are less likely to interact with eachother; thus, their functions hardly interfere with each other.Therefore, an increase in the driving voltage can be prevented.

The electron-relay layer contains a substance having a highelectron-transport property and is formed so that the LUMO level of thesubstance having a high electron-transport property is located betweenthe LUMO level of the acceptor substance contained in the compositematerial layer and the LUMO level of the substance having a highelectron-transport property contained in the electron-transport layer.In the case where the electron-relay layer contains a donor substance,the donor level of the donor substance is controlled so as to be locatedbetween the LUMO level of the acceptor substance in the compositematerial layer and the LUMO level of the substance having a highelectron-transport property contained in the electron-transport layer.As a specific value of the energy level, the LUMO level of the substancehaving a high electron-transport property contained in theelectron-relay layer is preferably greater than or equal to −5.0 eV,more preferably greater than or equal to −5.0 eV and less than or equalto −3.0 eV.

It is preferable to use a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand as thesubstance having a high electron-transport property contained in theelectron-relay layer.

As the phthalocyanine-based material contained in the electron relaylayer, specifically, any of the following is preferably used: CuPc, aphthalocyanine tin(II) complex (SnPc), a phthalocyanine zinc complex(ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanine iron(FePc), and vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine(PhO-VOPc).

As the metal complex having a metal-oxygen bond and an aromatic ligand,which is contained in the electron-relay layer, a metal complex having ametal-oxygen double bond is preferably used. The metal-oxygen doublebond has an acceptor property (a property of easily acceptingelectrons); thus, electrons can be transferred (donated and accepted)more easily. Further, the metal complex which has a metal-oxygen doublebond is considered stable. Thus, the use of the metal complex having themetal-oxygen double bond makes it possible to drive the light-emittingelement at low voltage more stably.

A phthalocyanine-based material is preferable as a metal complex havinga metal-oxygen bond and an aromatic ligand. Specifically, any of vanadylphthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex (SnOPc),and a phthalocyanine titanium oxide complex (TiOPc) is preferablebecause a metal-oxygen double bond is more likely to act on anothermolecular in teens of a molecular structure and an acceptor property ishigh.

Note that a phthalocyanine-based material having a phenoxy group ispreferable as the phthalocyanine-based materials described above.Specifically, a phthalocyanine derivative having a phenoxy group, suchas PhO-VOPc, is preferable. A phthalocyanine derivative having a phenoxygroup is soluble in a solvent. Thus, a phthalocyanine derivative has anadvantage of being easily handled during formation of the light-emittingelement and an advantage of facilitating maintenance of an apparatusused for forming a film.

The electron-relay layer may further contain a donor substance. As thedonor substance, any of the following can be used: an organic compoundsuch as tetrathianaphthacene (abbreviation: TTN), nickelocene, anddecamethylnickelocene, in addition to an alkali metal, an alkaline earthmetal, a rare earth metal, and a compound of the above metals (e.g., analkali metal compound (e.g., an oxide such as lithium oxide, a halide,and a carbonate such as lithium carbonate or cesium carbonate), analkaline earth metal compound (e.g., an oxide, a halide, and acarbonate), and a rare earth metal compound (e.g., an oxide, a halide,and a carbonate)). When such a donor substance is contained in theelectron-relay layer, electrons can be transferred easily and thelight-emitting element can be driven at lower voltage.

In the case where a donor substance is contained in the electron-relaylayer, in addition to the materials described above as the substancehaving a high electron-transport property, a substance having a LUMOlevel greater than the acceptor level of the acceptor substancecontained in the composite material layer can be used. Specifically, itis preferable to use a substance having a LUMO level of greater than orequal to −5.0 eV, preferably greater than or equal to −5.0 eV and lessthan or equal to −3.0 eV. As examples of such a substance, a perylenederivative and a nitrogen-containing condensed aromatic compound aregiven. Note that a nitrogen-containing condensed aromatic compound ispreferably used for the electron-relay layer because of its stability.

Specific examples of the perylene derivative include3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA),3,4,9,10-perylenetettracarboxylic-bis-benzimidazole (abbreviation:PTCBI), N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide(abbreviation: PTCDI-C8H), andN,N′-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (Hex PTC).

Specific examples of the nitrogen-containing condensed aromatic compoundinclude pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT(CN)₆),2,3-diphenylpyrido[2,3-b]pyrazine (2PYPR), and2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (F2PYPR).

Other than the above, 7,7,8,8-tetracyanoquinodimethane (abbreviation:TCNQ), 1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation:NTCDA), perfluoropentacene, copper hexadecafluoro phthalocyanine(abbreviation: F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylicdiimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene(abbreviation: DCMT), a methanofullerene (e.g., [6,6]-phenyl C₆₁ butyricacid methyl ester), or the like can be used.

Note that in the case where a donor substance is contained in theelectron-relay layer, the electron-relay layer may be formed by a methodsuch as co-evaporation of the substance having a high electron-transportproperty and the donor substance.

Each of the hole-injection layer, the hole-transport layer, the secondlight-emitting layer 122, and the electron-transport layer may be formedusing any of the materials given above. However, a material differentfrom the light-emitting material of the first light-emitting layer 120is preferably used as a light-emitting material for the secondlight-emitting layer 122.

The electrode 112 having a light-transmitting property is provided onthe side where light is extracted, and thus is formed using alight-transmitting material. As the light-transmitting material, indiumoxide, indium tin oxide, indium oxide-zinc oxide, zinc oxide, zinc oxideto which gallium is added, graphene, or the like can be used.

As the electrode 112 having a light-transmitting property, a metalmaterial such as gold, platinum, nickel, tungsten, chromium, molybdenum,iron, cobalt, copper, palladium, or titanium can be used. A nitride ofthe metal material (e.g., titanium nitride) or the like may be used. Inthe case of using the metal material (or the nitride thereof), theelectrode 112 having a light-transmitting property may be thinned so asto have a light-transmitting property.

The half width of an emission spectrum of each light-emitting layer inthe above-described light-emitting element illustrated in FIG. 1A can bereduced, so that the light-emitting element can have a sharp emissionspectrum. In addition, the light-emitting element can have highefficiency.

FIG. 1B illustrates a structure of a light-emitting element which isdifferent from the structure of the light-emitting element illustratedin FIG. 1A. The structure illustrated in FIG. 1B has many parts incommon with the structure of the light-emitting element illustrated inFIG. 1A. Therefore, in the following description, the same parts willnot be described.

The light-emitting element illustrated in FIG. 1B includes the electrode102 having a reflective property, and the first EL layer 106, a firstcharge generation layer 108, the second EL layer 110, a second chargegeneration layer 114, a third EL layer 118, and the electrode 112 havinga light-transmitting property which are stacked in this order over theelectrode 102 having a reflective property. The light-emitting elementillustrated in FIG. 1B is different from the light-emitting elementillustrated in FIG. 1A in that the second charge generation layer 114and the third EL layer 118 are provided between the second EL layer 110and the electrode 112 having a light-transmitting property. The otherparts of the structure are the same as those of the light-emittingelement illustrated in FIG. 1A.

In the light-emitting element illustrated in FIG. 1B, the third EL layer118 includes at least the third light-emitting layer 124. Light emittedfrom the third light-emitting layer 124 is extracted outside thelight-emitting element through the electrode 112 having alight-transmitting property. Components of light entering the electrode112 having a light-transmitting property include light entering theelectrode 112 having a light-transmitting property directly from thethird light-emitting layer 124 (the light is also referred to as thirdentering light) and light which is emitted from the third light-emittinglayer 124 and is reflected back by the electrode 102 having a reflectiveproperty (the light is also referred to as third reflected light). Thethird entering light and the third reflected light interfere with eachother. Thus, the optical path length between the electrode 102 having areflective property and the third light-emitting layer 124 is adjusted,whereby the phase of the third entering light and the phase of the thirdreflected light can be aligned with each other.

In FIG. 1B, the optical path length between the electrode 102 having areflective property and the third light-emitting layer 124 isfive-quarters of the peak wavelength of the emission spectrum of thethird light-emitting layer. This allows the phases of the third enteringlight and the third reflected light to be aligned and emission from thethird light-emitting layer 124 to be increased. Thus, the luminance ofthe light-emitting element according to this embodiment is higher thanthe luminance of a light-emitting element in which the optical pathlength is not adjusted, in the case where the same current is applied tothese light-emitting elements.

The optical path length between the electrode 102 having a reflectiveproperty and the third light-emitting layer 124 (the optical path lengthof the third reflected light) can be adjusted by adjusting the thicknessof the second charge generation layer 114. Note that the first chargegeneration layer 108 and the second charge generation layer 114 can havea structure similar to that of the charge generation layer 107 describedabove.

The third EL layer 118 may include at least the third light-emittinglayer 124. The description of the first EL layer 106 or the second ELlayer 110 may be referred to for the specific structure of the third ELlayer 118. Note that a light-emitting material different from thelight-emitting material of the first light-emitting layer 120 and thelight-emitting material of the second light-emitting layer 122 ispreferably used as a light-emitting material for the thirdlight-emitting layer 124.

Note that the structure of the light-emitting element according to thisembodiment is not limited to the structures illustrated in FIGS. 1A and1B. For example, a light-emitting element may include four or more ELlayers as illustrated in FIG. 2. Each EL layer includes at least alight-emitting layer. Moreover, it is preferable that a chargegeneration layer be provided between one EL layer and another EL layer.

In the structure of the light-emitting element illustrated in FIG. 2,the optical path length between the electrode 102 having a reflectiveproperty and a light-emitting layer (the optical path length ofreflected light) can be adjusted by adjusting the thickness of a chargegeneration layer.

In FIG. 2, for example, an n-th EL layer 218 (n is an integer of 4 ormore) includes an m-th light-emitting layer 224 (m is an integer of 4 ormore). In addition, an (n−1)th charge generation layer 214 is providedin contact with the n-th EL layer 218. In FIG. 2, the optical pathlength between the electrode 102 having a reflective property and them-th light-emitting layer 224 is set to (2m−1)/4 of the peak wavelengthof the emission spectrum of the m-th light-emitting layer 224. Thisallows the phases of m-th entering light and m-th reflected light to bealigned and emission from the m-th light-emitting layer 224 to beincreased. Thus, the luminance of the light-emitting element accordingto this embodiment is higher than the luminance of a light-emittingelement in which the optical path length is not adjusted, in the casewhere the same current is applied to these light-emitting elements.

FIGS. 3A to 3C each illustrate one embodiment of an EL display device inwhich a light-emitting element according to this embodiment. FIG. 3A isa plan view of a structure of an electrode of a display portion in thedisplay device. Note that some components (e.g., an EL layer) areillustrated in a simple way in FIGS. 3A to 3C for easy understanding.FIG. 3B is a cross-sectional view illustrating an example of a structureof the display portion. FIG. 3C is a cross-sectional view illustratinganother example of a structure of the display portion.

The display devices in FIGS. 3A to 3C are passive matrix displaydevices. In each of the display devices, the electrodes 102 havingreflective properties processed in stripes (a first electrode 102 ahaving a reflective property, a second electrode 102 b having areflective property, and a third electrode 102 c having a reflectiveproperty) and the electrodes 112 having light-transmitting propertiesprocessed in stripes (a first electrode 112 a having alight-transmitting property, a second electrode 112 b having alight-transmitting property, and a third electrode 112 c having alight-transmitting property) are stacked to form a lattice.

Layers (e.g., a first EL layer, a charge generation layer, and a secondEL layer) provided between the electrode 102 having a reflectiveproperty and the electrode 112 having a light-transmitting property areeach provided as a continuous film over an entire area between theelectrode 102 having a reflective property and the electrode 112 havinga light-transmitting property. Thus, selective deposition using a metalmask is not needed in a manufacturing process, which makes it possibleto perform film formation over a large area at one time, to increase thesize of the display device, and to improve the productivity. Moreover,the area of a display region in a display portion can be increased.Furthermore, defects due to the entry of particles, or the like whichoccurs when a metal mask is used can be prevented; thus, the displaydevice can be manufactured with high yields.

The display device illustrated in FIG. 3B includes a first pixel 130 aand a second pixel 130 b. The first pixel 130 a includes a firstlight-emitting element 132 a provided over a substrate 100 and a firstcolor filter layer 134 a provided for a counter substrate 128 in aregion overlapping with the first light-emitting element 132 a. Thesecond pixel 130 b includes a second light-emitting element 132 bprovided over the substrate 100 and a second color filter layer 134 bprovided for the counter substrate 128 in a region overlapping with thesecond light-emitting element 132 b.

The first light-emitting element 132 a includes a first conductive layer104 a having a light-transmitting property, the first EL layer 106, thecharge generation layer 107, the second EL layer 110, and the electrode112 having a light-transmitting property which are stacked in this orderover the first electrode 102 a having a reflective property. The secondlight-emitting element 132 b includes a second conductive layer 104 bhaving a light-transmitting property, the first EL layer 106, the chargegeneration layer 107, the second EL layer 110, and the electrode 112having a light-transmitting property which are stacked in this orderover the second electrode 102 b having a reflective property. Thelight-emitting element illustrated in FIG. 1A can be used as the firstlight-emitting element 132 a and the second light-emitting element 132b.

Note that the first electrode 102 a having a reflective property, thesecond electrode 102 b having a reflective property, the firstconductive layer 104 a having a light-transmitting property, and thesecond conductive layer 104 b having a light-transmitting property canbe processed into a desired shape in a photolithography step and anetching step. Thus, a fine pattern can be formed with goodcontrollability, which makes it possible to obtain a high-definitiondisplay device.

Further, when the first conductive layer 104 a having alight-transmitting property and the second conductive layer 104 b havinga light-transmitting property are provided independently in each pixel,crosstalk can be prevented even in the case where the thickness of theconductive layer having a light-transmitting property is extremely largeor the case where the conductivity of the conductive layer having alight-transmitting property is high.

An insulating layer 126 having openings is formed over the firstconductive layer 104 a having a light-transmitting property and thesecond conductive layer 104 b having a light-transmitting property. Thefirst EL layer 106 is in contact with the first conductive layer 104 ahaving a light-transmitting property and the second conductive layer 104b having a light-transmitting property through the openings. Theinsulating layer 126 is formed using an organic insulating material suchas polyimide, acrylic, polyamide, or epoxy, or an inorganic insulatingmaterial. It is particularly preferable that the insulating layer 126 beformed using a photosensitive resin material to have an opening overeach of the first conductive layer 104 a having a light-transmittingproperty and the second conductive layer 104 b having alight-transmitting property so that the sidewall of the opening isformed to have a tilted surface with continuous curvature. Theinsulating layer 126 may be tapered or inversely tapered.

Plastic (organic resin), glass, quartz, or the like can be used for thesubstrate 100. As an example of plastic, a member made of polycarbonate,polyarylate, polyethersulfone, or the like can be given. Plastic ispreferably used for the substrate 100, in which case a reduction in theweight of the display device can be achieved. Alternatively, a sheetwith a high barrier property against water vapor and a high heatradiation property (e.g., a sheet including diamond like carbon (DLC))can be used for the substrate 100.

Although not illustrated, a structure in which an inorganic insulator isprovided over the substrate 100 may be employed. The inorganic insulatorfunctions as a protective layer or a sealing film which blocks anexternal contaminant such as water. By providing the inorganicinsulator, deterioration of the light-emitting element can besuppressed; thus, the durability and lifetime of the display device canbe improved.

A single layer or a stack of a nitride film and a nitride oxide film canbe used as the inorganic insulator. Specifically, the inorganicinsulator can be formed using silicon oxide, silicon nitride, siliconoxynitride, aluminum oxide, aluminum nitride, aluminum oxynitride, orthe like by a CVD method, a sputtering method, or the like depending onthe material. It is preferable that the inorganic insulator be funnedusing silicon nitride by a CVD method. The thickness of the inorganicinsulator may be greater than or equal to 100 nm and less than or equalto 1 μm. Alternatively, an aluminum oxide film, a DLC film, a carbonfilm containing nitrogen, or a film containing zinc sulfide and siliconoxide (ZnS.SiO₂ film) may be used as the inorganic insulator.

Alternatively, a thin glass substrate can be used as the inorganicinsulator. For example, a glass substrate with a thickness greater thanor equal to 30 μm and less than or equal to 100 μm can be used.

A metal plate may be provided on a bottom surface of the substrate 100(a surface opposite to the surface over which the light-emitting elementis provided). In the case where an inorganic insulator is provided, ametal plate may be used instead of the substrate 100. Although there isno particular limitation on the thickness of the metal plate, a metalplate with a thickness greater than or equal to 10 μm and less than orequal to 200 μm is preferably used, in which case a reduction in theweight of the display device can be achieved. Further, although there isno particular limitation on the material of the metal plate, a metalsuch as aluminum, copper, or nickel, a metal alloy such as an aluminumalloy or stainless steel, or the like can be preferably used.

The metal plate and the substrate 100 can be bonded to each other withan adhesive layer. As the adhesive layer, a visible light curableadhesive, an ultraviolet curable adhesive, or a thermosetting adhesivecan be used. As examples of materials of such adhesives, an epoxy resin,an acrylic resin, a silicone resin, a phenol resin, and the like can begiven. A moisture-absorbing substance serving as a desiccant may becontained in the adhesive layer.

A metal plate has low permeability; thus, by providing the metal plate,the entry of moisture into the light-emitting element can be prevented.Consequently, by providing the metal plate, a highly reliable displaydevice in which deterioration due to moisture is suppressed can beprovided.

Note that an inorganic insulating film which covers the firstlight-emitting element 132 a and the second light-emitting element 132 bmay be provided. The inorganic insulating film serves as a protectivelayer or a sealing film which blocks an external contaminant such aswater. By providing the inorganic insulating film, the deterioration ofthe light-emitting element can be suppressed; thus, the durability andlifetime of the display device can be improved. A material similar tothe material of the inorganic insulator described above can be used as amaterial of the inorganic insulating film.

A moisture-absorbing substance which serves as a desiccant may beprovided between the substrate 100 and the counter substrate 128. Themoisture-absorbing substance may be provided in a solid state such aspowdery state or may be provided in a state of a film containing amoisture-absorbing substance over the first light-emitting element 132 aand the second light-emitting element 132 b by a film formation methodsuch as a sputtering method.

A material similar to that of the substrate 100 can be used for thecounter substrate 128. Note that the counter substrate 128 needs to havea property of transmitting light passing through at least the firstcolor filter layer 134 a and the second color filter layer 134 b.

For example, a chromatic color light-transmitting resin can be used asthe first color filter layer 134 a and the second color filter layer 134b. As the chromatic color light-transmitting resin, a photosensitiveorganic resin or a non-photosensitive organic resin can be used. Thephotosensitive organic resin is preferably used, in which case thenumber of resist masks can be reduced, which results in thesimplification of the process.

Chromatic colors are all colors except achromatic colors such as black,gray, and white. The color filter layer is formed using a material whichtransmits only light of the chromatic colors. As chromatic color, red,green, blue, or the like can be used. Alternatively, cyan, magenta,yellow, or the like may be used. “Transmitting only light of a chromaticcolor” means that light passing through the color filter layer has apeak at a wavelength of the light of the chromatic color.

The thickness of the color filter layer may be controlled to be optimalas appropriate in consideration of the relationship between theconcentration of a coloring material to be contained and thetransmittance of light. By making the color of the first color filterlayer 134 a and the color of the second color filter layer 134 bdifferent from each other, a display device capable of multicolordisplay can be obtained. Note that the color of light emitted from thefirst light-emitting layer 120 is preferably the same as the color ofthe first color filter layer 134 a and that the color of the lightemitted from the second light-emitting layer 122 is preferably the sameas the color of the second color filter layer 134 b.

In the display device described in this embodiment, the half width of aspectrum of light emitted from the first light-emitting layer 120 can bereduced by adjusting the optical path length between the first electrode102 a having a reflective property and the first light-emitting layer120 and by utilizing light interference. In a similar manner, the halfwidth of a spectrum of light emitted from the second light-emittinglayer 122 can be reduced by adjusting the optical path length betweenthe second electrode 102 b having a reflective property and the secondlight-emitting layer 122 and by utilizing light interference. Thus, whenthe first color filter layer 134 a and the light emitted from the firstlight-emitting layer have the same color, the concentration of acoloring material of the first color filter layer 134 a can be low. Inaddition, the thickness of the first color filter layer 134 a can besmall. The same can be applied to the second color filter layer 134 b.As a result, light absorption by the first color filter layer 134 a orthe second color filter layer 134 b can be reduced; thus, the useefficiency of light can be improved.

The example in which the first color filter layer 134 a and the secondcolor filter layer 134 b are provided on the inner side of the countersubstrate 128 is described in this embodiment. However, one embodimentof the present invention is not limited thereto. The first color filterlayer 134 a and the second color filter layer 134 b can be provided onthe outer side of the counter substrate 128 (i.e., on the opposite sideto the light-emitting elements).

Alternatively, a light-transmitting resin layer with a chromatic colorwhich functions as a color filter layer may be formed over the firstlight-emitting element 132 a and the second light-emitting element 132b.

A light-blocking layer may be provided in a region between the firstcolor filter layer 134 a and the second color filter layer 134 b (i.e.,a region overlapping with the insulating layer 126). The light-blockinglayer is formed using a light-blocking material which reflects orabsorbs light. For example, a black organic resin can be used, which canbe formed by mixing a black resin of a pigment material, carbon black,titanium black, or the like into a resin material such as photosensitiveor non-photosensitive polyimide. Alternatively, a light-blocking metalfilm can be used, which is made of chromium, molybdenum, nickel,titanium, cobalt, copper, tungsten, aluminum, or the like, for example.

There is no particular limitation on the formation method of thelight-blocking layer, and a dry method such as an evaporation method, asputtering method, or a CVD method, or a wet method such as a spincoating method, a dip coating method, a spray coating method, a dropletdischarge method (e.g., ink jetting), a screen printing method, or anoffset printing method may be used depending on the material. If needed,an etching method (dry etching or wet etching) may be employed to form adesired pattern.

The light-blocking layer can prevent light from leaking to an adjacentpixel. Therefore, by providing the light-blocking layer, an image can bedisplayed with high contrast and high definition.

The display device illustrated in FIG. 3C includes a first pixel 230 a,a second pixel 230 b, and a third pixel 230 c. The first pixel 230 aincludes a first light-emitting element 232 a provided over thesubstrate 100 and the first color filter layer 134 a provided for thecounter substrate 128 in a region overlapping with the firstlight-emitting element 232 a. A second pixel 230 b includes a secondlight-emitting element 232 b provided over the substrate 100 and thesecond color filter layer 134 b provided for the counter substrate 128in a region overlapping with the second light-emitting element 232 b. Athird pixel 230 c includes a third light-emitting element 232 c providedover the substrate 100 and a third color filter layer 134 c provided forthe counter substrate 128 in a region overlapping with the thirdlight-emitting element 232 c.

For example, the first color filter layer 134 a, the second color filterlayer 134 b, the third color filter layer 134 c are made to be blue,green, and red, respectively, whereby a display device capable offull-color display can be obtained.

The first light-emitting element 232 a includes the first electrode 102a having a reflective property, and the first conductive layer 104 ahaving a light-transmitting property, the first EL layer 106, the firstcharge generation layer 108, the second EL layer 110, the second chargegeneration layer 114, the third EL layer 118, and the electrode 112having a light-transmitting property which are stacked in this orderover the first electrode 102 a having a reflective property. The secondlight-emitting element 232 b includes the second electrode 102 b havinga reflective property, and the second conductive layer 104 b having alight-transmitting property, the first EL layer 106, the first chargegeneration layer 108, the second EL layer 110, the second chargegeneration layer 114, the third EL layer 118, and the electrode 112having a light-transmitting property which are stacked in this orderover the second electrode 102 b having a reflective property. The thirdlight-emitting element 232 c includes a third electrode 102 c having areflective property, and the third conductive layer 104 c having alight-transmitting property, the first EL layer 106, the first chargegeneration layer 108, the second EL layer 110, the second chargegeneration layer 114, and the third EL layer 118, and the electrode 112having a light-transmitting property which are stacked in this orderover the third electrode 102 c having a reflective property. Thelight-emitting element illustrated in FIG. 1B can be used as the firstlight-emitting element 232 a, the second light-emitting element 232 b,and the third light-emitting element 232 c.

In FIG. 3C, light emitted from the first light-emitting layer and thefirst color filter layer 134 a preferably have the same color; lightemitted from the second light-emitting layer and the second color filterlayer 134 b preferably have the same color; and light emitted from thethird light-emitting layer and the third color filter layer 134 cpreferably have the same color. This makes it possible to reduce lightabsorption by the first color filter layer 134 a, the second colorfilter layer 134 b, and the third color filter layer 134 c; thus, useefficiency of light can be improved.

In the display device described in this embodiment, the optical pathlength between the electrode having a reflective property and each ofthe plurality of light-emitting layers in the light-emitting elements isoptimized in accordance with the wavelength of light emitted from eachlight-emitting layer, whereby light of different colors can be extractedfrom pixels with high emission efficiency. Moreover, the half width ofan emission spectrum of each light-emitting layer is reduced; thus,light with high color purity can be extracted from the color filterlayer with high efficiency.

Furthermore, the light-emitting layer is formed as a continuous filmwithout performing selective deposition of light-emitting layers inpixels with the use of a metal mask. This can prevent a reduction inyield or a complicated process caused by the use of a metal mask. Thus,a high-definition and low-power-consumption display device can beprovided.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, an active matrix display device that is oneembodiment of the present invention will be described with reference toFIGS. 4A and 4B. FIG. 4A is a plan view illustrating a display device.FIG. 4B is a cross-sectional view taken along line A-B and C-D in FIG.4A.

In the display device illustrated in FIGS. 4A and 4B, an elementsubstrate 410 and a sealing substrate 404 are attached to each otherwith a sealant 405, and a driver circuit portion (a source-side drivercircuit 401 and a gate side driver circuit 403) and a pixel portion 402including a plurality of pixels are provided.

Note that a wiring 408 is a wiring for transmitting signals that are tobe inputted to the source side driver circuit 401 and the gate sidedriver circuit 403, and receives a video signal, a clock signal, a startsignal, a reset signal, and the like from a flexible printed circuit(FPC) 409 which serves as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The display device in this specification includes not only adisplay device itself but also a display device to which an FPC or a PWBis attached.

The driver circuit portion (the source side driver circuit 401 and thegate side driver circuit 403) includes a plurality of transistors. Aplurality of pixels included in the pixel portion 402 each include aswitching transistor, a current controlling transistor, and a firstelectrode electrically connected to a drain electrode of the currentcontrolling transistor.

Although the driver circuit portion (the source side driver circuit 401and the gate side driver circuit 403) and the pixel portion 402 areformed over the element substrate 410, FIG. 4B illustrates the sourceside driver circuit 401 which is the driver circuit portion and threepixels in the pixel portion 402.

The plurality of pixels in the pixel portion 402 each include theswitching transistor, the current controlling transistor, and the firstelectrode electrically connected to a drain electrode of the currentcontrolling transistor. The plurality of pixels include at least twopixels. In this embodiment, an example is described in which pixels ofthree colors, a blue (B) pixel 420 a, a green (G) pixel 420 b, and ared, (R) pixel 420 c, are provided.

The pixels 420 a, 420 b, and 420 c respectively include color filterlayers 434 a, 434 b, 434 c; light-emitting elements 418 a, 418 b, and418 c; and transistors 412 a, 412 b, and 412 c which are respectivelyelectrically connected to the light-emitting elements 418 a, 418 b, and418 c and function as switching transistors.

The light-emitting elements 418 a, 418 b, and 418 c respectively includea stacked layer of an electrode 413 a having a reflective property and aconductive layer 415 a having a light-transmitting property, a stackedlayer of an electrode 413 b having a reflective property and aconductive layer 415 b having a light-transmitting property, and astacked layer of an electrode 413 c having a reflective property and aconductive layer 415 c having a light-transmitting property. Inaddition, the light-emitting elements 418 a, 418 b, and 418 c include,over the respective stacked layers, a first EL layer 431 in which afirst light-emitting layer is provided, a first charge generation layer432, a second EL layer 433 in which a second light-emitting layer isprovided, a second charge generation layer 435, a third EL layer 436 inwhich a third light-emitting layer is provided, and an electrode 417having a light-transmitting property.

The distance between each of the electrodes 413 a, 413 b, and 413 chaving reflective properties and the first light-emitting layer isadjusted so as to be one-quarter of the peak wavelength of the emissionspectrum of the first light-emitting layer; the distance between each ofthe electrodes 413 a, 413 b, and 413 c having reflective properties andthe second light-emitting layer is adjusted by the first chargegeneration layer 432 so as to be three-quarters of the peak wavelengthof the emission spectrum of the second light-emitting layer; and thedistance between each of the electrodes 413 a, 413 b, and 413 c havingreflective properties and the third light-emitting layer is adjusted bythe second charge generation layer 435 so as to be five-quarters of thepeak wavelength of the emission spectrum of the third light-emittinglayer. The distance between the electrode having a reflective propertyand the first light-emitting layer can be adjusted by controlling thethickness of a thin film provided between the electrode having areflective property and the first light-emitting layer (e.g., aconductive layer having a light-transmitting property or a functionallayer included in the first EL layer). Moreover, the distance betweenthe electrode having a reflective property and the second light-emittinglayer or the third light-emitting layer can be adjusted by controllingthe thickness of the first charge generation layer 432 or the secondcharge generation layer 435.

A color filter layer which transmits light with a peak wavelength of theemission spectrum of the first light-emitting layer is used as the colorfilter layer 434 a; a color filter layer which transmits light with apeak wavelength of the emission spectrum of the second light-emittinglayer is used as the color filter layer 434 b; and a color filter layerwhich transmits light with a peak wavelength of the emission spectrum ofthe third light-emitting layer is used as the color filter layer 434 c.

For example, the color filter layer 434 a of the blue (B) pixel 420 amay be blue, the color filter layer 434 b of the green (G) pixel 420 bmay be green, and the color filter layer 434 c of the red (R) pixel 420c may be red.

By optimizing the distance between the electrode having a reflectiveproperty and each of the first light-emitting layer, the secondlight-emitting layer, and the third light-emitting layer in accordancewith the wavelength of color of light emitted from each light-emittinglayer, light of different colors can be extracted from the pixels withhigh emission efficiency. Moreover, the half width of an emissionspectrum of each light-emitting layer is reduced; thus, light with highcolor purity can be extracted from each of the color filter layers 434a, 434 b, and 434 c with high efficiency.

For example, the first light-emitting layer is made to emit red light,the second light-emitting layer is made to emit green light, and thethird light-emitting layer is made to emit blue light, whereby whitelight can be extracted from the display device without providing a colorfilter layer. Thus, a display device including pixels corresponding toR, B, and W can be manufactured. A light-emitting device which emitswhite light can also be manufactured. Moreover, the light-emittingdevice can be used as a lighting device.

Furthermore, the light-emitting layer is formed as a continuous filmwithout performing selective deposition of light-emitting layers inpixels with the use of a metal mask. This can prevent a reduction inyield or a complicated process caused by the use of a metal mask. Thus,a high-definition display device with high color reproducibility can beprovided. In addition, a low-power-consumption display device can beprovided.

A CMOS circuit, which is a combination of an n-channel transistor 423and a p-channel transistor 424, is formed for the source side drivercircuit 401. The driver circuit may be any of a variety of circuitsformed with transistors, such as a CMOS circuit, a PMOS circuit, or anNMOS circuit. Although the example in which the source side drivercircuit and the gate side driver circuit are formed over a substrate isdescribed in this embodiment, one embodiment of the present invention isnot limited thereto. All or part of the source side driver circuit andthe gate side driver circuit may be formed outside a substrate, not overthe substrate.

Note that an insulator 414 is formed to cover end portions of theelectrodes 413 a, 413 b, and 413 c having reflective properties and endportions of the conductive layers 415 a, 415 b, and 415 c havinglight-transmitting properties. Here, the insulator 414 is formed using apositive type photosensitive acrylic resin film.

In order to improve the coverage, the insulator 414 is provided suchthat either an upper end portion or a lower end portion of the insulator414 has a curved surface with a curvature. For example, when positivephotosensitive acrylic is used as a material for the insulator 414, itis preferable that only an upper end portion of the insulator 414 have acurved surface with a radius of curvature (0.2 μm to 3 μm). For theinsulator 414, it is also possible to use either a negative type thatbecomes insoluble in an etchant by light irradiation or a positive typethat becomes soluble in an etchant by light irradiation.

Any of the materials described in Embodiment 1 can be used for each ofthe color filter layers 434 a, 434 b, and 434 c, the electrodes 413 a,413 b, and 413 c having reflective properties, the conductive layers 415a, 415 b, and 415 c having light-transmitting properties, the first ELlayer 431, the first charge generation layer 432, the second EL layer433, the second charge generation layer 435, the third EL layer 436, andthe electrode 417 having a light-transmitting property.

The sealing substrate 404 is attached to the element substrate 410 withthe sealant 405; thus, a light-emitting element 418 is provided in aspace 407 enclosed by the element substrate 410, the sealing substrate404, and the sealant 405. Note that the space 407 is filled with afiller and may be filled with an inert gas (e.g., nitrogen or argon), anorganic resin, or the sealant 405. A substance having a hygroscopicproperty may be used as the organic resin and the sealant 405.

Note that an epoxy-based resin is preferably used as the sealant 405. Itis preferable that such a material allow as little moisture and oxygenas possible to penetrate. As a material for the sealing substrate 404, aglass substrate, a quartz substrate, or a plastic substrate made offiberglass-reinforced plastics (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used.

As in this embodiment, an insulating film 411 which serves as a basefilm may be provided between the element substrate 410 and asemiconductor layer of the transistor. The insulating film has afunction of preventing diffusion of an impurity element from the elementsubstrate 410 and can be formed to have a single-layer structure or astacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

There is no particular limitation on the structure of the transistorwhich can be used in the display device disclosed in this specification;for example, a staggered type transistor or a planar type transistorhaving a top-gate structure or a bottom-gate structure can be used. Thetransistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned over and below a channel region with a gate insulating layertherebetween.

The gate electrode layers can be formed to have a single-layer orstacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as itsmain component.

For example, as a two-layer structure of the gate electrode layer, thefollowing structures are preferable: a two-layer structure of analuminum layer and a molybdenum layer stacked thereover, a two-layerstructure of a copper layer and a molybdenum layer stacked thereover, atwo-layer structure of a copper layer and a titanium nitride layer or atantalum nitride layer stacked thereover, and a two-layer structure of atitanium nitride layer and a molybdenum layer. As a three-layerstructure, a three-layer structure in which a tungsten layer or atungsten nitride layer, an alloy of aluminum and silicon or an alloy ofaluminum and titanium, and a titanium nitride layer or a titanium layerare stacked is preferable.

The gate insulating layer can be formed to have a single-layer structureor a stacked-layer structure using a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, and/or a silicon nitrideoxide layer by a plasma CVD method, a sputtering method, or the like.Alternatively, a silicon oxide layer formed by a CVD method using anorganosilane gas can be used as the gate insulating layer. As anorganosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula:SiH(OC₂H₅)₃), or trisdimethylaminosilane (chemical formula:SiH(N(CH₃)₂)₃) can be used.

A material of the semiconductor layer is not particularly limited andmay be determined as appropriate in accordance with the characteristicsneeded for the transistors 412 a, 412 b, 412 c, 423, and 424. Examplesof materials which can be used for the semiconductor layer will begiven.

As the material of the semiconductor layer, any of the following can beused: an amorphous semiconductor manufactured by or a sputtering methodor a vapor-phase growth method using a semiconductor material gastypified by silane or germane; a polycrystalline semiconductor formed bycrystallizing the amorphous semiconductor with the use of light energyor thermal energy; and a microcrystalline semiconductor. Thesemiconductor layer can be formed by a sputtering method, an LPCVDmethod, a plasma CVD method, or the like.

A single crystal semiconductor made of silicon or silicon carbide can beused for the semiconductor layer. When a single crystal semiconductor isused for the semiconductor layer, the size of the transistor can bereduced; thus, higher resolution pixels in a display portion can beobtained. In the case where a single crystal semiconductor is used forthe semiconductor layer, an SOI substrate in which a single crystalsemiconductor layer is provided can be used. Alternatively, asemiconductor substrate such as a silicon wafer may be used.

A typical example of an amorphous semiconductor is hydrogenatedamorphous silicon, and a typical example of a crystalline semiconductoris polysilicon and the like. Examples of polysilicon (polycrystallinesilicon) include so-called high-temperature polysilicon which containspolysilicon formed at a process temperature of 800° C. or higher as itsmain component, so-called low-temperature polysilicon which containspolysilicon formed at a process temperature of 600° C. or lower as itsmain component, and polysilicon obtained by crystallizing amorphoussilicon with the use of an element that promotes crystallization.Needless to say, a microcrystalline semiconductor or a semiconductorpartly containing a crystal phase can be used as described above.

Further, an oxide semiconductor may be used. As the oxide semiconductor,the following can be used: an oxide of four metal elements such as anIn—Sn—Ga—Zn—O-based oxide semiconductor; an oxide of three metalelements such as an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxidesemiconductor; or an oxide of two metal elements such as anIn—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor,an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxidesemiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-basedoxide semiconductor, or In—Ga—O-based oxide semiconductor; an In—O-basedoxide semiconductor; a Sn—O-based oxide semiconductor; or a Zn—O-basedoxide semiconductor. Further, SiO₂ may be contained in the above oxidesemiconductor. Here, for example, an In—Ga—Zn—O-based oxidesemiconductor is an oxide containing at least In, Ga, and Zn, and thecomposition ratio of the elements is not particularly limited. TheIn—Ga—Zn—O-based oxide semiconductor may contain an element other thanIn, Ga; and Zn.

A thin film expressed by a chemical formula of InMO₃(ZnO)_(m)(m>0) canbe used for the oxide semiconductor layer. Here, M represents one ormore metal elements selected from Ga, Al, Mn, and Co. For example, M canbe Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, the atomic ratio thereof is In/Zn=0.5 to 50, preferablyIn/Zn=1 to 20, further preferably In/Zn=1.5 to 15. When the atomic ratioof Zn is in the above preferred range, the field-effect mobility of atransistor can be improved. Here, when the atomic ratio of the compoundis In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y is satisfied.

For the oxide semiconductor layer, a crystalline oxide semiconductorwhich has neither a single crystal structure nor an amorphous structureand has c-axis alignment (also referred to as c-axis aligned crystalline(CAAC) oxide semiconductor).

As examples of materials for a wiring layer serving as a sourceelectrode layer or a drain electrode layer, the following are given: anelement selected from Al, Cr, Ta, Ti, Mo, and W; an alloy containing anyof the above elements as its component; an alloy film containing acombination of any of these elements; and the like. In the case whereheat treatment is performed, a conductive film preferably has heatresistance high enough to withstand the heat treatment. Since the use ofAl alone brings disadvantages such as low heat resistance and a tendencyfor corrosion, aluminum is used in combination with a conductivematerial having heat resistance. As the conductive material having heatresistance, which is combined with Al, it is possible to use an elementselected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloycontaining any of these elements as its component, an alloy containing acombination of any of these elements, or a nitride containing any ofthese elements as its component.

An inorganic insulating film or an organic insulating film formed by adry method or a wet method can be used for an insulating film 419 whichcovers the transistors. For example, a silicon nitride film, a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, atantalum oxide film, or a gallium oxide film which is formed by a CVDmethod, a sputtering method, or, the like can be used. Alternatively, anorganic material such as polyimide, acrylic, benzocyclobutene,polyamide, or an epoxy resin can be used. Other than the above organicmaterials, a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like can be used.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include, as a substituent, anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.The organic group may include a fluoro group. A siloxane-based resin isapplied by a coating method and baked; thus, the insulating film 419 canbe formed.

Note that the insulating film 419 may be formed by stacking a pluralityof insulating films formed using any of the above-described materials.For example, a structure may be employed in which an organic resin filmis stacked over an inorganic insulating film.

In the above manner, the active matrix display device including thelight-emitting element of one embodiment of the present invention can beobtained.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

A display device disclosed in this specification can be applied to avariety of electronic appliances (including game machines). Examples ofelectronic devices include a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio reproducing device, and alarge-sized game machine such as a pachinko machine.

FIG. 5A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying the display device described in Embodiment 1 or 2to the display portion 3003, the laptop personal computer can have ahigh level of definition and consumes a small amount of power.

FIG. 5B illustrates a personal digital assistant (PDA), which includes adisplay portion 3023, an external interface 3025, an operation button3024, and the like in a main body 3021. The personal digital assistantalso includes a stylus 3022 as an accessory for operation. By applyingthe display device described in Embodiment 1 or 2 to the display portion3023, the personal digital assistant (PDA) can have a high level ofdefinition and consumes a small amount of power.

FIG. 5C illustrates an e-book reader, which includes two housings, ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the e-book reader can be openedand closed with the hinge 2711 as an axis. Such a structure enables thee-book reader to operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 5C) and graphics can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 5C). By applying thedisplay device described in Embodiment 1 or 2 to the display portion2705 and the display portion 2707, the e-book reader can have a highlevel of definition and consumes a small amount of power. In the casewhere a semi-transmissive display device or a reflective display deviceis used for the display portion 2705, a solar battery may be provided sothat the solar battery can generate power and a battery can be chargedfor the use in relatively bright conditions. Note that when a lithiumion battery is used as the battery, an advantage such as reduction insize can be obtained.

Further, FIG. 5C illustrates an example in which the housing 2701 isprovided with an operation portion and the like. For example, thehousing 2701 is provided with a power switch 2721, operation keys 2723,a speaker 2725, and the like. Pages can be turned with the operation key2723. Note that a keyboard, a pointing device, or the like may also beprovided on the surface of the housing, on which the display portion isprovided. Furthermore, an external connection teiuiinal (an earphoneterminal, a USB terminal, or the like), a recording medium insertionportion, and the like may be provided on the back surface or the sidesurface of the housing. Further, the e-book reader may have a functionof an electronic dictionary.

The e-book reader may wirelessly transmit and receive data. Throughwireless communication, desired book data or the like can be purchasedand downloaded from an electronic book server.

FIG. 5D illustrates a mobile phone, which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing. 2800 includes a solar cell 2810 having a functionof charge of the mobile phone, an external memory slot 2811, and thelike. Further, an antenna is incorporated in the housing 2801. Byapplying the display device described in Embodiment 1 or Embodiment 2 tothe display panel 2802, the mobile phone can have a higher level ofdefinition and consumes a smaller amount of power.

The display panel 2802 is provided with a touch panel. A plurality ofoperation keys 2805 which are displayed as images are illustrated bydashed lines in FIG. 5D. Note that a boosting circuit by which voltageoutput from the solar cell 2810 is increased to be sufficiently high foreach circuit is also included.

The display direction in the display panel 2802 is changed asappropriate depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802; thus, it can be used as a video phone. The speaker 2803 andthe microphone 2804 can be used for videophone calls, recording andplaying sound, and the like without limitation to voice calls. Moreover,the housing 2800 and the housing 2801 developed as illustrated in FIG.5D can be slid so that one is lapped over the other; thus, the size ofthe mobile phone can be reduced, which makes the mobile phone suitablefor being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, an infrared communication function, a television receptionfunction, or the like may be provided in addition to the abovefunctions.

FIG. 5E illustrates a digital video camera, which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Byapplying the display device described in Embodiment 1 or Embodiment 2 toeach of the display portion A 3057 and the display portion B 3055, thedigital video camera can have a higher level of definition and consumesa smaller amount of power.

FIG. 5F illustrates a television device in which a display portion 9603and the like are incorporated in a housing 9601. Images can be displayedon the display portion 9603. Here, the housing 9601 is supported by astand 9605. By applying the display device described in Embodiment 1 orEmbodiment 2 to the display portion 9603, the television device can havea higher level of definition and consumes a smaller amount of power.

The television device can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Note that the structure described in this embodiment can be combinedwith the structure described in Embodiment 1 or 2 as appropriate.

Example

In this example, measurement results of characteristics of alight-emitting element according to one embodiment of the presentinvention will be described with reference to a drawing and tables.

In this example, a manufacturing method of the light-emitting elementwill be described with reference to FIG. 6.

Shown below are structural formulae of organic compounds used in thisexample (BPhen, 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: PCzPA), 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene(abbreviation: CzPA), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6 mMemFLPAPrn),2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBA1BP), and(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)).

As an electrode 1101 having a reflective property of the light-emittingelement, an aluminum-nickel-lanthanum alloy film was formed over asubstrate 1100 which was a glass substrate by a sputtering method. Inthis example, the electrode 1101 having a reflective property was usedas an anode. Note that the electrode area was 2 mm×2 mm.

Next, indium tin oxide containing silicon oxide (ITSO) was depositedover the electrode 1101 having a reflective property by a sputteringmethod to form a conductive layer 1104 having a light-transmittingproperty with a thickness of 10 nm.

Next, the substrate 1100 provided with the electrode 1101 having areflective property and the conductive layer 1104 having alight-transmitting property was fixed to a substrate holder provided ina vacuum evaporation apparatus such that the surface on which theelectrode 1101 having a reflective property and the conductive layer1104 having a light-transmitting property were formed faced downward,and then the pressure was reduced to about 10⁻⁴ Pa. After that, PCzPAand molybdenum(VI) oxide were co-evaporated on the conductive layer 1104having a light-transmitting property to form a hole-injection layer1111. The weight ratio of PCzPA to molybdenum oxide was adjusted to be1:0.5 (=PCzPA: molybdenum oxide). The thickness of the hole-injectionlayer 1111 was 10 nm. Note that the co-evaporation method refers to anevaporation method in which evaporation is carried out from a pluralityof evaporation sources at the same time in one treatment chamber.

Next, PCzPA was deposited to a thickness of 25 nm on the hole-injectionlayer 1111 to form a hole-transport layer 1112.

On the hole-transport layer 1112, CzPA and 1,6mMemFLPAPrn wereco-evaporated so that the weight ratio of CzPA to 1,6mMemFLPAPrn was1:0.05 (=CzPA: 1,6mMemFLPAPrn) to form a light-emitting layer 1113. Thethickness of the light-emitting layer 1113 was 30 nm.

On the light-emitting layer 1113, CzPA was deposited to a thickness of 5nm to form an electron-transport layer 1114 a.

On the electron-transport layer 1114 a, bathophenanthroline(abbreviation: BPhen) was deposited to a thickness of 15 nm to form anelectron-transport layer 1114 b.

On the electron-transport layer 1114 b, lithium oxide (Li₂O) wasevaporated to a thickness of 0.1 nm to form an electron-injection layer1115 a, and on the electron-injection layer 1115 a, copper(II)phthalocyanine (abbreviation: CuPc) was evaporated to a thickness of 2nm to form an electron-injection layer 1115 b.

On the electron-injection layer 1115 b, PCzPA and molybdenum(VI) oxidewere co-evaporated to form a charge generation layer 1102. The weightratio of PCzPA to molybdenum oxide was adjusted to be 1:0.5 (=PCzPA:molybdenum oxide). The thickness of the charge generation layer 1102 was120 nm.

On the charge generation layer 1102, BPAFLP was deposited to a thicknessof 20 nm to form a hole-transport layer 1212.

On the hole-transport layer 1212, 2mDBTPDBq-II, PCBA1BP, andIr(mppr-Me)₂(acac) were co-evaporated so that the weight ratio of2mDBTPDBq-II to PCBA1BP and Ir(mppr-Me)₂(acac) was 0.6:0.2:0.06(=2mDBTPDBq-II: PCBA1BP: Ir(mppr-Me)₂(acac)) to form a light-emittinglayer 1213. The thickness of the light-emitting layer 1213 was 15 nm.

On the light-emitting layer 1213, 2mDBTPDBq-II and Ir(mppr-Me)₂(acac)were co-evaporated so that the weight ratio of 2mDBTPDBq-II toIr(mppr-Me)₂(acac) was 1:0.06 (=2mDBTPDBq-II: Ir(mppr-Me)₂(acac)) toform a light-emitting layer 1313. The thickness of the light-emittinglayer 1313 was 15 nm.

On the light-emitting layer 1313, 2mDBTPDBq-II was deposited to athickness of 20 nm to form an electron-transport layer 1214 a.

On the electron-transport layer 1214 a, BPhen was deposited to athickness of 10 nm to form an electron-transport layer 1214 b.

On the electron-transport layer 1214 b, lithium oxide (Li₂O) wasevaporated to a thickness of 0.1 nm to form an electron-injection layer1215 a, and on the electron-injection layer 1215 a, copper(II)phthalocyanine (abbreviation: CuPc) was evaporated to a thickness of 2nm to form an electron-injection layer 1215 b.

On the electron-injection layer 1215 b, BPAFLP and molybdenum(VI) oxidewere co-evaporated to form a charge generation layer 1105. The weightratio of BPAFLP to molybdenum oxide was adjusted to be 1:0.5 (=BPAFLP:molybdenum oxide). The thickness of the charge generation layer 1105 was20 nm.

On the charge generation layer 1105, indium tin oxide (ITO) wasdeposited to a thickness of 110 nm by a sputtering method to form anelectrode 1103 having a light-transmitting property.

Through the above steps, the light-emitting element of this example wasmanufactured.

Note that in all of the above evaporation steps, a resistance heatingmethod was employed.

Table 1 shows the element structure of the light-emitting element ofthis example, which was manufactured in the above manner.

TABLE 1 1101 1104 1111 1112 1113 1114a light- Al—Ni—La ITSO PCzPA:MoOxPCzPA CzPA:1,6- CzPA emitting 10 nm (=1:0.5) 25 nm mMemFLPA 5 nm element10 nm Prn (=1:0.05) 30 nm 1114b 1115a 1115b 1102 1212 1213 BPhen Li₂OCuPc PCzPA:MoOx BPAFLP 2mDBTPDBq- 15 nm 0.1 nm 2 nm (=1:0.5) 20 nmII:PCBA1BP:Ir(mppr- 120 nm Me)₂acac (=0.6:0.2:0.06) 15 nm 1313 1214a1214b 1215a 1215b 1105 1103 2mDBTPDBq-II:Ir(mppr- 2mDBTPD BPhen Li₂OCuPc BPAFLP:MoOx ITO Me)₂acac Bq-II 10 nm 0.1 nm 2 nm (=1:0.5) 110 nm(=1:0.06) 20 nm 20 nm 15 nm

The light-emitting element was sealed with a glass substrate in a glovebox under a nitrogen atmosphere so as not to be exposed to the air.

FIG. 7 shows the emission spectrum of the light-emitting element. InFIG. 7, the horizontal axis represents wavelength (nm) and the verticalaxis represents emission intensity (arbitrary unit). As shown in FIG. 7,the emission spectrum of the light-emitting element has a peak around465 nm, which is attributed to the light-emitting layer 1113, and a peakaround 586 nm, which is attributed to the light-emitting layer 1213. Awavelength of 465 nm was obtained as the peak of the emission spectrumof the light-emitting layer 1113. A wavelength of 586 nm was obtained asthe peak of the emission spectrum of the light-emitting layer 1213.

The optical path length between the electrode 1101 having a reflectiveproperty and the light-emitting layer 1113 in the light-emitting elementof this example is one-quarter of the peak wavelength (465 nm) of theemission spectrum of the light-emitting layer 1113. The optical pathlength between the electrode 1101 having a reflective property and thelight-emitting layer 1213 in the light-emitting element of this exampleis three-quarters of the peak wavelength (586 nm) of the emissionspectrum of the light-emitting layer 1213.

The optical path length is calculated by the following formula:refractive index×length (thickness). Table 2 shows the thickness andrefractive index of each layer, which were used for calculating theoptical path length of the light-emitting element; one-quarter of thepeak wavelength (465 nm) of the emission spectrum of the light-emittinglayer 1113; the refractive index of the light-emitting layer 1113; andthe calculated optical path lengths.

TABLE 2 Thickness Refractive index at Optical path (nm) around 465 nmlength (nm) ITSO 10 2.18 21.8 PCzPA-OMOx 10 1.91 19.1 PCzPA 25 1.92 48Total 88.9 ¼ of peak wavelength (465 nm) of emission 116.25 spectrum oflight-emitting layer 1113 Refractive index of 1.86 light-emitting layer1113 (CzPA) Light-emitting region (distance from interface 14.7 betweenhole-transport layer 1112 and light- emitting layer 1113 tolight-emitting region)

Table 2 shows that the optical path length between the electrode 1101having a reflective property and a light-emitting region of thelight-emitting layer 1113, which is located at about 14.7 nm from aninterface with the hole-transport layer 1112, corresponds to one-quarterof the peak wavelength (465 nm) of the emission spectrum of thelight-emitting layer 1113.

Table 3 shows the thickness and refractive index of each layer, whichwere used for calculating the optical path length of the light-emittingelement; three-quarters of the peak wavelength (586 nm) of the emissionspectrum of the light-emitting layer 1213; the refractive index of thelight-emitting layer 1213; and the calculated optical path lengths.

TABLE 3 Thickness Refractive index at Optical path (nm) around 586 nmlength (nm) ITSO 10 2.18 21.8 PCzPA-OMOx 10 1.91 19.1 PCzPA 25 1.92 48Light-emitting 30 1.78 53.4 layer 1113 (CzPA) CzPA 5 1.78 8.9 BPhen 151.7 25.5 CuPc 2 1.45 2.9 PCzPA-OMOx 120 1.83 219.6 BPAFLP 20 1.74 34.8Total 430.4 ¾ of peak wavelength (586 nm) of emission 439.5 spectrum oflight-emitting layer 1213 Refractive index of 1.78 light-emitting layer1213 (2mDBTPDBq-II) Light-emitting region (distance from interface 5.11between hole-transport layer 1212 and light- emitting layer 1213 tolight-emitting region)

Table 3 shows that the optical path length between the electrode 1101having a reflective property and a light-emitting region of thelight-emitting layer 1213, which is located at about 5.11 nm from aninterface with the hole-transport layer 1212, corresponds tothree-quarters of the peak wavelength (586 nm) of the emission spectrumof the light-emitting layer 1213.

At a luminance of 914 cd/m², the CIE chromaticity coordinates of thelight-emitting element were (x=0.46, y=0.37), which indicates favorablewhite light emission. Further, at a luminance of 913 cd/m², the currentefficiency was 68.0 cd/A, the external quantum efficiency was 26.8%, thevoltage was 6.2 V, the current density was 1.3 mA/cm², and the powerefficiency was 34.4 lm/W.

The light-emitting element of this example has favorable lightdistribution characteristics in accordance with the Lambertian radiationpattern. This confirmed that the light-emitting element of this exampleis useful as a planar light source.

This application is based on Japanese Patent Application serial no.2011-027960 filed with the Japan Patent Office on Feb. 11, 2011, theentire contents of which are hereby incorporated by reference.

1. A light-emitting element comprising: an electrode having a reflective property; a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; a second light-emitting layer over the first charge generation layer; and an electrode having a light-transmitting property over the second light-emitting layer, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, and wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer.
 2. The light-emitting element according to claim 1, wherein colors of light emitted from the first light-emitting layer and the second light-emitting layer are different.
 3. The light-emitting element according to claim 1, wherein the light-emitting element emits white light.
 4. The light-emitting element according to claim 1, further comprising: a first substrate provided with the electrode having a reflective property; and a second substrate over the electrode having a light-transmitting property, wherein the second substrate is a light-transmitting substrate.
 5. The light-emitting element according to claim 1, further comprising: a second charge generation layer over the second light-emitting layer; and a third light-emitting layer between the second charge generation layer and the electrode having a light-transmitting property, wherein an optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of a peak wavelength of an emission spectrum of the third light-emitting layer.
 6. The light-emitting element according to claim 5, wherein colors of light emitted from the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer are different.
 7. The light-emitting element according to claim 1, further comprising: a hole-injection layer between the electrode having a reflective property and the first light-emitting layer; and a second charge generation layer between the second light-emitting layer and the electrode having a light-transmitting property, wherein the hole-injection layer comprises a first organic compound and an inorganic compound, wherein the first charge generation layer comprises the first organic compound and the inorganic compound, and wherein the second charge generation layer comprises a second organic compound and the inorganic compound.
 8. A light-emitting device comprising the light-emitting element according to claim
 1. 9. A lighting device comprising the light-emitting element according to claim
 1. 10. A light-emitting device comprising: a first light-emitting element, the first light-emitting element comprising: an electrode having a reflective property; a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; a second light-emitting layer over the first charge generation layer; and an electrode having a light-transmitting property over the second light-emitting layer; a first color filter layer over the first light-emitting element; a second light-emitting element adjacent to the first light-emitting element; and a second color filter layer over the second light-emitting element, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer, wherein the first color filter layer transmits light with a peak wavelength of the emission spectrum of the first light-emitting layer, and wherein the second color filter layer transmits light with a peak wavelength of the emission spectrum of the second light-emitting layer.
 11. The light-emitting device according to claim 10, wherein colors of light emitted from the first light-emitting layer and the second light-emitting layer are different.
 12. The light-emitting device according to claim 10, further comprising: a first substrate provided with the first light-emitting element and the second light-emitting element; and a second substrate provided with the first color filter layer and the second color filter layer, wherein the second substrate is a light-transmitting substrate.
 13. The light-emitting device according to claim 10, further comprising: a hole-injection layer between the electrode having a reflective property and the first light-emitting layer; and a second charge generation layer between the second light-emitting layer and the electrode having a light-transmitting property, wherein the hole-injection layer comprises a first organic compound and an inorganic compound, wherein the first charge generation layer comprises the first organic compound and the inorganic compound, and wherein the second charge generation layer comprises a second organic compound and the inorganic compound.
 14. A light-emitting device according to claim 10, wherein the light-emitting device is a display device.
 15. A light-emitting device comprising: a plurality of light-emitting elements, the plurality of light-emitting elements each comprising: an electrode having a reflective property; a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; a second light-emitting layer over the first charge generation layer; a second charge generation layer over the second light-emitting layer; a third light-emitting layer over the second charge generation layer; an electrode having a light-transmitting property over the third light-emitting layer; and a plurality of color filter layers provided over the plurality of light-emitting elements, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer, wherein an optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of a peak wavelength of an emission spectrum of the third light-emitting layer, and wherein the plurality of color filter layers comprise at least: a first color filter layer transmitting light with a peak wavelength of the emission spectrum of the first light-emitting layer; a second color filter layer transmitting light with a peak wavelength of the emission spectrum of the second light-emitting layer; and a third color filter layer transmitting light with a peak wavelength of the emission spectrum of the third light-emitting layer.
 16. The light-emitting device according to claim 15, wherein colors of light emitted from the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer are different.
 17. The light-emitting device according to claim 15, further comprising: a first substrate provided with the plurality of light-emitting elements; and a second substrate provided with the plurality of color filter layers, wherein the second substrate is a light-transmitting substrate.
 18. A light-emitting device according to claim 15, wherein the light-emitting device is a display device. 